feat: add Docker Compose setup with bind-mount appdata directory

- Dockerfile builds image with app source in /app_src
- docker-entrypoint.sh syncs code from image on each start, inits DB
- docker-compose.yml mounts ./appdata:/app for easy backup
- backend/init_db.py for safe DB init (no drop_all)
- backend/database.py supports DATABASE_URL env var
- .dockerignore excludes venv, __pycache__, *.db, .git

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -0,0 +1,10 @@
+services:
+  app:
+    build: .
+    ports:
+      - "8000:8000"
+    volumes:
+      - ./appdata:/app
+    environment:
+      - DATABASE_URL=sqlite:////app/data/app.db
+    restart: unless-stopped